The invention relates, generally, to chemical mechanical planarizing (CMP) processes for removing barrier metals and, more particularly, to polishing compositions for selectively removing barrier metals in the presence of interconnect structures in integrated circuit devices.
In recent years, the semiconductor industry has increasingly relied upon copper electrical interconnects in forming integrated circuits. These copper interconnects have a low electrical resistivity and a high resistance to electromigration. Since copper is very soluble in many dielectric materials, such as silicon dioxide and low-K or doped versions of silicon dioxide, a diffusion barrier layer is necessary to prevent the diffusion of copper into the underlying dielectric material. Typical barrier materials include, tantalum, tantalum nitride, tantalum-silicon nitrides, titanium, titanium nitrides, titanium-silicon nitrides, titanium-titanium nitrides, titanium-tungsten, tungsten, tungsten nitrides and tungsten-silicon nitrides.
In response to increasing demands for high density integrated circuits, manufacturers now fabricate integrated circuits containing multiple overlying layers of metal interconnect structures. During device fabrication, planarizing each interconnect layer improves packing density, process uniformity, product quality and most importantly, enables chip manufacturers to fabricate multiple layer integrated circuits. Chip manufacturers rely upon chemical-mechanical-planarizing (CMP) as a cost effective means of producing flat substrate surfaces. The CMP process is typically carried out in a two-step sequence. First, the polishing process uses a “first-step” slurry specifically designed to rapidly remove copper. For example, Carpio et al., in “Initial study on copper CMP slurry chemistries” Thin Solid Films, 262 (1995), disclose the use on a 5 weight percent nitric acid solution for efficient copper removal. Similarly, Kondo et al., in U.S. Pat. No. 6,117,775, disclose the use of nitric acid and BTA for copper removal.
After the initial copper removal, a “second-step” slurry removes the barrier material. Typically, second-step slurries require excellent selectivity to remove the barrier material without adversely impacting the physical structure or electrical properties of the interconnect structure.
Because it was traditionally perceived that alkaline polishing slurries have a much higher Ta/TaN removal rate than acidic slurries, commercial second-step slurries typically have a basic to neutral pH. Another factor pointing to the advantages of neutral to basic pH barrier metal polishing slurries relates to the need to preserve the metal overlying the barrier metal during the second-step polishing. The metal removal rate should be very low to reduce dishing of the metal interconnects.
In acidic slurries that include oxidizing agents, copper tends to have both a high removal rate and a high static etch rate. Cote et al. however, in U.S. Pat. No. 6,375,693, disclose an acidic CMP slurry for barrier materials. The slurry of Cote et al. operates with a hydrogen peroxide oxidizer, a benzotriazole inhibitor and a sulfated fatty acid at a pH range of 2 to 7.5. Similarly, Wojtczak et al., in U.S. Pat. No. 6,409,781, disclose an acidic polishing slurry that relies upon a potassium iodate oxidizer, iminodiacetic acid as the copper corrosion inhibitor and nitric acid as the copper activator to selectively polish the barrier material.
Future low k and ultra-low k integrations of the IC structure will require low metal and dielectric losses in the CMP step. Accordingly, a selective slurry for barrier removal will most probably be adopted. While neutral-to-basic polishing slurries have advantages known to those skilled in the art, such as those described above, these slurries also tend to have low tantalum removal rates. In addition, because tantalum is readily oxidized, the oxidizing agents in the slurry can react with the tantalum to form an oxide layer on the surface. In view of the above, there exists a need to provide a second-step slurry that possesses both a rapid dissolution of barrier materials and excellent selectivity to both interconnect metals and dielectric materials.